Just bought a Mokai, education required

Discussion in 'Jet Drives' started by curtis73, Oct 18, 2015.

  1. curtis73
    Joined: Mar 2002
    Posts: 80
    Likes: 1, Points: 8, Legacy Rep: 14
    Location: Variable, North America

    curtis73 Junior Member

    I have owned dozens of boats from 12' to 27', but never owned a jet. Until now.

    This Mokai has a Honda GX190, so I have a feeling with my 250-lb butt in it, it will hit 10 mph at best. I have seen some common upgrades to get these to 20+ mph (Robin EX21 with an EX17 head, pickup grate mods, etc) but haven't found much more than that. Likely due to the proprietary pump setup and obvious heat issues associated with an air-cooled engine in a small engine room.

    Where I would use this Mokai would be in two different places; one is a 13-mile long winding lake and the other is a pretty swift river with mild whitewater. So my options are to either make it faster or just sell it and rethink.

    I understand basic principles of jet drives, but I have a very prop-based understanding. I have good fabrication and technical skills, so fitting a different engine isn't an issue, but where I lack experience is in these two categories: 1) how to assess what speeds the hull will handle, and 2) how to assess how much the pump will handle.

    I know that I won't generate any more thrust unless I spin the current impeller faster, or use an impeller that moves more mass, but how much faster can I spin it efficiently? How do I figure out the limits of the housing and volume it can move? Are there resources that might help narrow it down; like compressor maps for turbos?

    I suppose what I'm asking for is some jet theory 101.
  2. Speedling
    Joined: May 2016
    Posts: 10
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    Location: Cedar Lake

    Speedling Junior Member

    More power doesn't mean more speed unless you can increase the pitch of the impeller or you can decrease the nozzle bore which will increase the velocity of the water leaving the jet nozzle. If there is room put in a Honda gx390 and then decrease nozzle bore by creating inserts or you can also see if there is a bearing cone and build inserts for that to extend it thus making the exit nozzle have less area thus increasing velocity for the same volume thus increase of speed. If you have the power that is.
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